1. Field of the Invention
The present invention relates generally to implantable medical devices, and more particularly, to an implantable medical device including surface geometry having reduced biofilm formation characteristics.
2. Related Art
One of the more common reasons to explant an implantable medical device (IMD) is the development of a chronic infection associated with the device after implantation into the recipient. This reason for explant has become of more importance in recent times due to the increased reliability of many types of IMDs. One infection source is thought to be the formation of biofilms on a surface region of an IMD. These biofilms occur when microbes colonize a surface and form a film or “slime” layer. This mode of microbe life is now considered more common than the more generally studied planktonic state of bacteria. Studies have shown that in the biofilm state, microbes or bacteria within the biofilm are protected by an extracellular matrix structure which also assists in nutrition. (See, e.g., Peters G et al., Adherence and growth of coagulate-negative staphylococci on surfaces of intravenous catheters. J Infect Dis 146:479-482 (1982); Gray E D et al, Effect of extracellular slime substance from Staphylococcus epidermidis on human cellular immune response. Lancet 18:365-367 (1984); Johnson G M et al, Interference with granulocyte function by Staphylococcus epidermidis slime. Infect Immun 54: 13-20 (1986); Kaplan S S et al, Biomaterial associated impairment of local neutrophil function. ASAIO Trans 36:M172-175 (1990); and Anwar H et al, Dynamic interactions of biofilms of mucoid Pseudomonas aeruginosa with tobramycin and piperacillin. Antimicrob Agents Chemother 36: 1208-1214 (1992).) In addition, microbes in the biofilm state express different genes than in the planktonic state. (See, e.g., Costerton J W et al., The Application of Biofilm Science to the Study and Control of Chronic Bacterial Infections, J Clin Invest 112: 1466-1477 (2003).) The process by which microbes form a biofilm on a surface region is now generally well accepted. The first step in this process is the attaching of free moving (planktonic) microbes to the relevant surface region. In subsequent steps, the attached microbes then colonize the surface and then develop into a mature biofilm. (See, e.g., Costerton J W, The Biofilm Primer (Springer Series on Biofilms), pp 5-7 (2007); and Mack D, Molecular mechanisms of Staphylococcus epidermidis biofilm formation. J Hosp Infect 43 SupphSl 13-125 (1999).)
The formation of a biofilm is thought to behave as a physical and/or chemical barrier to antimicrobial agents and the body's natural defenses against infection. Clinicians have observed that antibiotics have not been effective against infections resulting from biofilms despite assaying showing the component microbe of the biofilm to be of a type normally susceptible to antibiotics. (See, e.g., Costerton J W, The Biofilm Primer (Springer Series on Biofilms), pp 56-61 (2007).) Additionally, studies have shown that microbes in the biofilms state are up to a thousand times more resistant to antibiotics than in the planktonic state. (See, e.g., Jass, J., Surman, S., Walker, J. ‘Medical Biofilms: Detection, Prevention and Control’ Wiley, Chichester, p 7, (2003); Nickel et al, Antibiotic resistance of Pseudomonas aeruginosa colonising a urinary catheter in vitro., European Journal of Clinical Microbiology, 4:213-218 (1985).) Accordingly, should a biofilm form on a surface region of an IMD then it is more likely that this will result in a chronic infection, thereby resulting in the otherwise effective IMD having to be removed.
There have been a number of attempts to reduce the formation of biofilms on the surface regions of IMDs to reduce this instance of chronic infection. One approach involves the use of an electric field or current applied to the surface and surrounding regions to disrupt biofilm formation. This approach typically rquires a power supply to form the relevant electric field. This may necessitate the incorporation of a power supply into the IMD in the case where the IMD is passive, or alternatively placing an extra load requirement on an active powered IMD such as a cochlear implant or cardio-stimulator device.
Other approaches to reducing the formation of biofilms on the surface regions of IMDs involve the use of enzymes or other antimicrobial agents as a coating on the relevant surface region of the IMD. However, the use of any antimicrobial agent or biocide may be susceptible to the adaptive abilities of any biofilm to develop a resistance to the therapeutic properties of the agent, thereby potentially rendering it not as effective as it otherwise might be. Furthermore, for IMDs which are intended for long term implantation, the effectiveness of these coatings may diminish over extended time periods. Additionally, the process of coating an IMD with a suitable antimicrobial agent or biocide also adds complexity and cost to the manufacture of IMDs as it generally involves the coating of unsuitable substrates such as silicone or may involve a high temperature process which the electronic components of an IMD cannot withstand.